The invention relates to a method for controlling the power supply of a mobile data memory that performs wireless data transfer with a read/write device. The mobile data memory has at least one energy accumulator and other consumers of the energy. The invention further relates to a mobile data memory and an identification system with at least one read/write device and a mobile data memory.
GB 2 284 728 discloses a data communication device for wireless data transfer. Such a data communication device which has a control mechanism for the power supply of the data communication device.
In conventional art, identification systems are known, which have one or more stationary read/write devices, which exchange with mobile data memories via a data transfer link using in a wireless fashion. The data exchange is usually radio-based. Such identification systems are commonly used in technical installations where objects or goods must be moved as quickly and freely as possible. These objects are of various types. For example, e.g., packages in a shipping installation, assembly parts in a production line, luggage in a transport system, etc., need to be moved as quickly and freely as possible.
An example of such an identification system is described in the ISO/IEC JTC 1/SC 31 WG4 Draft Standard entitled “Radio Frequency Identification Standard for Item Management—Air Interface, (WD 18000) dated Aug. 15, 2001.
According to the above draft standard, the read/write device polls for the presence of a mobile data memory in the detection range. The read/write device performs the polling by emitting an unmodulated first carrier signal with a predefinable backscatter frequency. The frequency could be, for example, 2.45 GHz. A mobile data memory located in the reception range passively returns this signal to the read/write device, e.g., by backscattering.
Independently thereof, the mobile data memory modulates the impedance of an integrated transmit/receive antenna in cyclic sequences with a significant identification sequence to identify the mobile data memory in a read/write device. In addition, the read/write device receives time information as to when the mobile data memory switches on its data receiver. If the read/write device can receive the returned modulated backscatter frequency, the validity of the reply is checked. If the reply is determined to be valid, the read-write device, in addition, applies a communication frequency to the carrier signal at the time when the mobile data memory is expected to be ready for receiving.
In the above proposed standard, the communication frequency is shifted relative to the backscatter frequency by a fixed frequency amount, e.g., by approximately 10.6 MHz. By applying the communication frequency, the read/write device signals to the mobile data memory that a data transfer will follow. The second signal with the communication frequency is therefore modulated with data for performing the data transfer.
The transferred data can contain, for example, an identification number of the read/write device. The received signal from the transmit/receive antenna of the mobile data memory is polled cyclically and at short intervals for the presence of the backscatter frequency and the communication frequency. If both frequencies are detected, the data receiver of the mobile data memory remains enabled to receive data.
The mobile data memory usually has an energy accumulator, particularly a battery, for receiving its required supply of power. To enhance the service life of the mobile data memory, it becomes necessary to minimize the power consumption. One conventional way of reducing the power consumption, for example, is to select electronic components that have particularly low power requirements.
Furthermore, the design of a mobile data memory must take into account that a far greater amount of energy is required to receive data than to transmit data. In contrast to the very low power requirement of the initially described passive backscattering in which the antenna impedance is modulated only momentarily, several circuits must be connected for data reception. In addition, these circuits, e.g., data modulators, controllers and electronic memories, require a minimum period before transients subside and the components are ready for operation.
Furthermore, data is generally transferred between the read/write device and the mobile data memories only during a fraction of the total operating period. The above-cited draft standard has proposed, therefore, that the data receiver of the mobile data memory be activated only cyclically to reduce energy consumption. Typically, the pulse duty factor of ON to OFF time is approximately 0.3 to 0.4. During the ON time, the validity of any date present is checked.
Despite the above-described measures, conventional art does not provide techniques for minimizing the power consumption sufficient enough so that is no need to replace the battery during the service life of the mobile data memory. This increases the possibility of the data being stored in the mobile data memory being lost if the battery is exhausted.
A further drawback is that the mobile data memory has to be withdrawn, for example, from the production process to change the battery in time. This can cause delays and can interfere with the process flow. Depending on the application, the required life of a mobile data carrier can be approximately 10 to 15 years.